In a recent article published in Nature Communications, we proposed several revisions to the taxonomy of the bacterial order Caulobacterales1. To meet the criteria for an ‘effective publication’ according to the International Code of Nomenclature of Prokaryotes (ICNP), we here provide protologues for the proposed taxonomic revisions, which were originally included in the electronic supplementary material rather than the main article (see Note 1 to Rule 25a of the ICNP)2. Motivations for these proposals are based on our phylogenomic analyses and are found in the ‘Supplementary Note 2’ of Hallgren et al.1.

Microbial communities are structured through complex interactions that are difficult to observe directly. Co-occurrence networks offer a way to infer community structure, revealing (not exclusively) potential biotic interactions. Such networks have been inferred for diverse biomes and repeatedly found to be modular, yet the ecological significance of this modularity remains underexplored. We tested whether clusters within co-occurrence networks (“cohorts”), are universal and ecologically meaningful units by assessing their ubiquity, stability, and environmental specificity across diverse ecosystems. Our meta-analysis spans 25 previously published 16S rRNA gene amplicon sequencing datasets (14 160 samples) and covers high environmental variability ranging from aquatic, terrestrial to anthropogenic environments. Microbial co-occurrence networks consistently exhibited high modularity across biomes. Inferred cohorts were ubiquitous and represented up to 90% of the community composition. Our findings demonstrate that modularity is a fundamental and generalizable feature of microbial community organization, indicating the existence of stable subcommunities. Highly similar cohorts were inferred even across different, unconnected environments and datasets, and showed consistent responses to environmental gradients, indicating that their composition is to a large degree deterministic and predictable. The overall cohort structure and environmental preferences were independent of the sample size and the inference algorithm, underlining the robustness and applicability of the results. Recognizing these microbial cohorts as a meaningful level of microbial organization will refine microbial community ecology, cultivation strategies, and predictive modelling of microbial dynamics.

Fungi play essential roles across ecosystems, yet their diversity in aquatic environments remains poorly understood compared to terrestrial systems. To address this gap, we analyzed metagenomes from 26 lakes in the boreal and subarctic zones, along with one tropical reservoir, to characterize fungal and fungal-like (Oomycota) community structure. We also examined environmental factors shaping these communities. Most variation in fungal composition was explained by lake identity, depth layer, and season, with total organic carbon as a significant explanatory variable. Despite geographic and time differences, dominant fungal phyla, orders, and genera were largely consistent across all lakes. However, genus-level variation indicated distinct community compositions likely influenced by differences in carbon substrate availability. Attempts to classify metagenomic reads down to the species level—illustrated here through the well-characterized oomycete genus Phytophthora—were constrained by the limited taxonomic resolution of current reference databases. While metagenomics offers powerful means to investigate entire microbial communities, our results underscore a persistent bottleneck: the insufficient representation of aquatic eukaryotic genomes in public databases.

CO2 fixation (i.e. primary production) is a key function of all ecosystems, providing the carbon and energy that fuel the entire food web. It also plays an important role in mitigating climate change as CO2 is the most important greenhouse gas. While photosynthesis is regarded as the most important carbon fixation pathway, prokaryotes able to fix carbon in the absence of light (chemolithoautotrophs) can also be a significant source of energy in a light-limited ecosystem. Boreal lakes, notoriously colored and stratified with respect to oxygen and nutrients, present ideal conditions for this so-called dark carbon fixation by the chemolithoautotrophs. However, the prevalence of dark carbon fixation in boreal lakes remains unknown. Here, we measured dark carbon fixation in Swedish lakes from the boreal and boreo-nemoral zones, during summer stratification. We detected dark carbon fixation in 16 out of the 17 lakes studied, and concluded that dark fixation is a widespread phenomenon in boreal lakes. Moreover, the average dark primary production ranged from 18.5 % in the epilimnion to 81.4 % in the hypolimnion of all tested lakes. Our data further suggests that chemolithoautotrophic activity is mostly driven by iron-oxidizing bacteria. The chemolithoautotrophic guild is diverse and seems to be composed of both ubiquitous bacteria, like Gallionellaceae or Chromatiaceae, and endemic taxa, such as Ferrovaceae, which appears to be favored by a low pH. These results are particularly exciting as they suggest that dark carbon fixation could partly compensate for the low photosynthetic capacity in lakes with dark-colored water.

Introduction  Human microbiota-associated (HMA) models are used to allow in vivo studies of the human gut microbiome and its effects on host physiology. In particular, alterations in early life microbiota have been linked to allergy development during childhood. In this study, we investigated how pools of human microbiota collected from infants with different allergy risk, thrive in mice and their offspring, as well as how they influence the host metabolome.

Method  We used a two-generation HMA mouse model in which dams were colonized with human feces from three groups of infants (n = 19, samples collected during the first 8 weeks of life). In two of the groups, all infants had a strong hereditary risk for allergic disease (n = 12), but only 6 of them developed allergy before 2 years of age. In the third group, which was used as a control, none of the infants had allergic heredity or developed allergy (n = 7). Microbiota trajectories were followed from inoculation to mouse offspring, and metabolic profiles were monitored in several intestinal organs as well as in the serum of the murine offspring.

Results  The human microbiota adapted to the murine host but still presented distinct compositional features, reflecting the original inoculated samples. These microbial differences were mirrored in the mouse offspring metabolome, with group-associated patterns in sphingolipids, acylcarnitines and tryptophan metabolites. Furthermore, the metabolic profiles of the mouse offspring aligned with those observed in fecal water preparations from the corresponding human infant fecal samples.

Conclusion  Our findings highlight the significant impact of early-life microbiota on the host metabolome and show that our two-generation HMA model is suitable for studying microbiota‒metabolome relationships relevant to humans. The differences in microbiota‒metabolome correlations between individuals who develop or do not develop allergic disease suggest that an allergic predisposition might be more multifaceted than previously believed.

Microorganisms thrive in complex communities shaped by intricate interactions, yet the extent and ecological implications of biosynthetic dependencies in natural communities remain underexplored. Here, we used a dilution approach to cultivate 204 microbial model communities from the Baltic Sea and recovered 527 metagenome-assembled genomes (MAGs) that dereplicated into 72 species-clusters (>95% average nucleotide identity, ANI). Of these species, at least 70% represent previously uncultivated lineages. Combined with 1073 MAGs from Baltic Sea metagenomes, we generated a genomic catalog of 701 species-clusters. Our results show that cultures with more than three species included microorganisms with smaller genome sizes, lower biosynthetic potential for amino acids and B vitamins, and higher prevalence and abundance in the environment. Moreover, the taxa found together in the same model communities had complementary biosynthetic gene repertoires. Our results demonstrate that cultivating bacteria in dilution model communities facilitates access to previously uncultivated but abundant species that likely depend on metabolic partners for survival. Together, our findings highlight the value of community-based cultivation for unraveling ecological strategies. Finally, we confirm that metabolic interdependencies and genome streamlining are widespread features of successful environmental microorganisms.

Gut microbiomes of holometabolous insects can be strongly affected by metamorphosis. Previous studies suggest that microbiome colonization and community development often rely on specialized transmission routes between host life stages. However, there is a lack of comparative studies of microbial community dynamics from different transmission mechanisms. We compared the gut microbial community dynamics across life stages in five Galerucella species that differ in their potential microbial transfer mechanism by sequencing amplicons of the 16S rRNA gene. Females of three of the studied species place a fecal string on top of the egg, which may enhance the transfer of gut microbes, whereas females of the two other species do not. We found that the α-diversity was more stable between life stages in fecal string-placer species compared with the non-fecal string-placer species. Moreover, there were consistent microbiome differences between species, with multiple taxa in each species consistently appearing in all life stages. Fecal strings placed on eggs seem to play an important role in the diversity and dynamics of gut bacteria in Galerucella species, facilitating the vertical transfer of gut bacteria between host insect generations. Alternative, but less efficient, transmission routes appear to occur in non-fecal string-placer species.

Microbial biotransformation of trace organic chemicals (TOrCs) is an essential process in wastewater treatment to eliminate environmental pollution. Understanding TOrC biotransformation mechanisms, especially at their original concentrations, is important to optimize treatment performance, whereas our current knowledge is limited. Here, we investigated the biotransformation of seven TOrCs by 24 model communities. The genome-centric analyses unraveled potential biotransformation drivers concerning functional genes, enzymes, and responsible bacteria. We obtained efficient model communities for completely removing ibuprofen, caffeine, and atenolol, with transformation efficiencies between 0 % and 45 % for sulfamethoxazole, carbamazepine, trimethoprim, and gabapentin. Biotransformation performance was not fully reflected by the presence of known biotransformation genes and enzymes in the metagenomes of the communities. Functional similar homologs to existing biotransformation genes and enzymes (e.g., long-chain-fatty-acid-CoA ligase encoded by fadD and fadD13 gene) could play critical roles in TOrC metabolism. Finally, we identified previously undescribed degrading strains, e.g., Rhodococcus qingshengii for caffeine, carbamazepine, sulfamethoxazole, and ibuprofen biotransformation, and potential transformation enzymes, e.g., SDR family oxidoreductase targeting sulfamethoxazole and putative hypothetical proteins for caffeine, atenolol and gabapentin biotransformation. This study provides fundamental insights into naturally assembled low-complexity degrader communities that can help to identify and tackle the current research gaps on biotransformation.

Ecology and evolution are considered distinct processes that interact on contemporary time scales in microbiomes. Here, to observe these processes in a natural system, we collected a two-decade, 471-metagenome time series from Lake Mendota (Wisconsin, USA). We assembled 2,855 species-representative genomes and found that genomic change was common and frequent. By tracking strain composition via single nucleotide variants, we identified cyclical seasonal patterns in 80% and decadal shifts in 20% of species. In the dominant freshwater family Nanopelagicaceae, environmental extremes coincided with shifts in strain composition and positive selection of amino acid and nucleic acid metabolism genes. These genes identify organic nitrogen compounds as potential drivers of freshwater responses to global change. Seasonal and long-term strain dynamics could be regarded as ecological processes or, equivalently, as evolutionary change. Rather than as distinct interacting processes, we propose a conceptualization of ecology and evolution as a continuum to better describe change in microbial communities.

Vitamin B₁₂ (B₁₂) is an essential cofactor for vital metabolic processes in both prokaryotes and eukaryotes. De novo B₁₂ biosynthesis is exclusively carried out by a modicum of prokaryotes, although being required by most organisms. Recently, it has been demonstrated that not all B₁₂-prototrophic bacteria voluntarily share this vital cofactor and, therefore, are termed B₁₂-retainers. Consequently, low biosynthesis potential and limited voluntary release lead to a large discrepancy between availability and demand for B₁₂ in the ocean, indicating that release of B₁₂ may be an important control. Hence, in this study, we examined a specific release process, cell lysis after phage infection. We isolated bacteriophages specific for the B₁₂-prototrophic, yet B₁₂-retainer bacterium Sulfitobacter sp. M39. The addition of the bacteriophages to a Sulfitobacter sp. M39 mono-culture led to a significant increase in virus-like particles, reduced bacterial growth, and quantifiable extracellular dissolved B₁₂. When introducing bacteriophages to a co-culture comprising the host bacterium and the B₁₂-auxotrophic diatom Thalassiosira pseudonana, we observed rapid response in the form of microalgal growth. Our results indicate that B₁₂ is released as a result of bacteriophage-mediated cell lysis of Sulfitobacter sp. M39, enabling the growth of T. pseudonana in co-culture and possibly other microbes in nature. Therefore, we propose that bacteriophage-mediated cell lysis is a key mechanism for the release of essential metabolites, including vitamins, and given the estimated bacteriophage infection rates in the ocean, it plays a crucial role in the B-vitamin cycle in the marine environment.